Effect of environmental novelty and electroconvulsive shock on the tail flick reflex after placebo or morphine pellet implants.

After implantation of either a placebo or morphine pellet, different groups of rats were either assessed or not assessed on the nociceptive tail-flick test. After 1 week, latencies of placebo-implanted rats were significantly increased if the animals were inexperienced with the nociceptive test or if they were unfamiliar with the environment in which the test was administered. Latencies were not altered by an electroconvulsive shock (ECS) administered 1 week before the reflex test. In contrast, latencies of morphine-pretreated rats were not altered by prior experience with the nociceptive test per se. However, latencies of animals with prior test experience were modified by environmental novelty and ECS. Rats who became tolerant to morphine in a familiar environment were hyperalgesic when subsequently tested in unfamiliar surroundings. The latencies of rats who received a single ECS during the development of tolerance were unchanged 1 week later, whereas the latencies of rats who did not receive ECS showed a significant decline during this interval. These data demonstrate the mutual contribution of pharmacological and environmental variables in nociceptive behavior.